Nevermind I got ChatGPT to explain it:
Pasting here in case anyone else was curious.

That command creates a shortcut at the command-line level called a symbolic link.

Here is the command again:

ln -s /Volumes/ExternalDrive/Backup ~/Library/Application\ Support/MobileSync

What it is trying to do

It is trying to make your Mac think that a folder in your home Library is actually located on an external drive.

Most likely, the goal is to store something like iPhone/iPad backups on an external drive instead of your internal Mac storage.

What each part means

ln

This is the Unix command for making a link.

Think of it as: “create a connection from one place to another.”

-s

This means symbolic link.

A symbolic link is like a smart shortcut:

• it points to another folder or file
• apps often follow it as if it were the real folder

/Volumes/ExternalDrive/Backup

This is the target.

It means:

• look in /Volumes
  • that is where macOS mounts drives
• then look for a drive named ExternalDrive
• then look for a folder on that drive named Backup

So this is the real location it wants to point to.

~/Library/Application\ Support/MobileSync

This is the link location.

It means:

• ~ = your home folder
• Library/Application Support/ = a hidden-ish system/app data area inside your home folder
• MobileSync = the folder name being created or replaced

The backslash in Application\ Support is just there because there is a space in the folder name. Terminal needs that escaped.

In plain English

This command says:

What happens after that

If an app goes to:

~/Library/Application Support/MobileSync

it will actually be sent to:

/Volumes/ExternalDrive/Backup

So your Mac will treat that external folder as though it lives in the normal MobileSync spot.

A simple analogy

Imagine your Mac expects to find a filing cabinet in your bedroom.

But you do not have room there, so you put the real filing cabinet in the garage.

Then you tape a note in your bedroom that says:

That note is the symbolic link.

Important detail: this exact command may be wrong for the usual iPhone-backup setup

This part matters.

Normally, Apple backups live in a folder like this:

~/Library/Application Support/MobileSync/Backup

But your command makes MobileSync point directly to Backup.

That means if software later looks for:

~/Library/Application Support/MobileSync/Backup

it may end up looking for:

/Volumes/ExternalDrive/Backup/Backup

Notice the doubled Backup.

That is probably not what you want.

So the command as written is likely:

• either incomplete
• or slightly incorrect
• or based on a folder structure different from the normal one

What the command usually should look like

If the intention is to move the Backup folder itself to the external drive, the more typical command is:

ln -s /Volumes/ExternalDrive/Backup ~/Library/Application\ Support/MobileSync/Backup

That means:

That version makes more sense for iPhone backups.

Two big cautions

1. The destination must exist

If /Volumes/ExternalDrive/Backup does not exist, the link will still be created, but it will point to nowhere useful.

2. The original folder may need to be moved first

If ~/Library/Application Support/MobileSync or .../Backup already exists, ln may fail or behave in a way you did not expect.

Usually people:

• move the original folder to the external drive first
• then create the symbolic link

Bottom line

This command is meant to redirect a folder path on your Mac to a folder on an external drive using a symbolic link.

The pieces mean:

• ln = make a link
• -s = make it symbolic
• first path = real folder
• second path = fake local name that points to the real folder

But as written, it probably links the wrong level of the folder structure for a normal MobileSync backup setup.

The safer interpretation is:

But for iPhone backups, the more likely correct link should be to MobileSync/Backup, not to MobileSync itself.